ASD Air Quality e375

8/3/2019 ASD Air Quality e375

1/12

Real Time Monitoring with

Integrated Control of Air Quality in Buildings

Ir K.F. Lau and Ir K.W. ChanArchitectural Services Department

Hong Kong SAR Government

SUMMARY

The levels of air quality parameters of a building are always changing with time. Airquality level signals from real-time monitoring instruments can be used to controlfunctioning of an air-conditioning system to respond to the real-time levels of air

quality parameters. This enables us to control the air conditioning to the levels of ourpre-set targets to avoid wastage of energy due to over provision. In comparison real-

time control with periodic sampling of air quality parameters, real-time control can

respond to the actual air quality so as to ensure achievement of the pre-set targets.

A proposal is made to classify the air quality parameters into three categories. The

Primary Parameters shall comprise of temperature, relative humidity, carbon dioxide,carbon monoxide, nitrogen dioxide and ventilation with positive air pressure. TheSecondary Parameters shall comprise of ozone, total volatile organic compound,

formaldehyde, and respirable suspended particles. The Tertiary Parameters shallcomprise of radon, air velocity and airborne bacteria.

Continuous monitoring of the profiles on the Primary Parameters can be integrated inthe automatic control of HVAC systems and also be effectively used in routine

building management. The need for continuous monitoring of the SecondaryParameters is at discretion of the building management. Measurement of the TertiaryParameters shall only be done under expert advice and supervision.

The importance of categorizing the indoor air quality parameters, the methodology of

real-time measurements of air quality parameters, and the application of real-time aircondition control are discussed.

Keywords: real-time air quality parameters control monitor


2/12

AIR QUALITY PROFILE BY REAL-TIME MONITORING

The levels of air quality parameters of a building are always changing with thepassage of time, and that periodic sampling and measurement of air quality

parameters is inadequate to safeguard good air conditioning without paying dueattention and recognition to the dynamics of changing concentration of theseparameters over time. With the help of Air Quality Profile[1] identified through the

use of by real time monitoring which depicts changes of air quality over a time period,we are able to observe visually exact timing and degree of deterioration or

improvement of air quality. Based on the Air Quality Profiles and utilizing real-timemonitors, integrated control system can be developed for controlling air-conditioningand ventilating a building to enable proper management if air quality in the built

environment.

THE AIR QUALITYPARAMETERS

We have found it prohibitively costly and time-consuming to monitor and assess thelevels of all air quality parameters with reproducible, reliable and conclusive results.

This is particularly true for monitoring the smaller indoor environments such as anoffice or a school assembly hall of under 500 m2 floor area. Such small spacesconstitute the majority of air-conditioned premises in Hong Kong. Taking an active

approach in solving the problem, we recommend to sub-divide the air qualityparameters into Primary, Secondary and Tertiary Parameters as follows.

Primary Parameters

The Primary Parameters shall include temperature, relative humidity (RH), carbondioxide (CO2), carbon monoxide (CO), and nitrogen dioxide (NO2). These are the

most prevalent and essential air quality parameters that affect the comfort and workefficiency of the occupants as well as their immediate and long-term health. Theseare our recommended air quality parameters for routine air condition monitoring.

They are the basic indicators of the state of health of the air quality in a building. Allof these Primary Parameters can be monitored by real-time monitor with alarm signals

integrated with the control of the central air-conditioning system.

We further recommend ventilation with positive air pressure to be considered as a

Primary Parameter, taking into consideration the necessity to prevent ingress of


3/12

by a source in the indoor environment, such as a photocopier, new furniture, carpet,tobacco smoke, etc.

Both ozone and formaldehyde have distinct recognizable odour and can cause

irritation tomucous membrane of the eyes, nose and throat. Ozone, being highly active and short-living, does not travel far before it is totally destroyed in the atmosphere.

Formaldehyde, being more stable then ozone, tends to accumulate in an environmentwith stagnant air. TVOC and RSP are common contaminants in premises with new

interior decoration. The levels of all Secondary Parameters can be controlled eitherby removal of the sources of pollutant or by improved ventilation, and on particularconditions, by isolation and decontamination.

Tertiary Parameters

The Tertiary Parameters shall include radon (Rn), airborne bacteria and airmovement.

Radon is an inherent pollutant generated by uranium bearing construction material.

Its presence in our indoor atmosphere is of such low levels that it can only be detected

on the radiation of its atomic decay. Being a noble gas eight times heavier than air,

and having very short half-life, radon gas is never evenly distributed in air whichrenders all short-term or grab sampling measurements meaningless.

Airborne bacteria cannot be monitored by real-time instruments, and their countscannot be correlated to public health and human comfort. Airborne aerobic bacteria

are usually non-pathogenic micro-organisms. Anaerobic bacteria are pathogenic butmost of them are easily killed by exposure to air. Virus, fungal spores, mites etc arenot classified as airborne bacteria, although they can be disease causing. As the

human body, not the air, usually is the main carrier of pathogenic diseases, bacteriacounts are often related to density of occupancy.

Air Movement is totally inconsistent from one testing point to another. Forcontrolling of air conditioning system, it is more appropriate to replace Air Velocity

by the Primary Parameter of Ventilation with positive indoor pressure as the later ismore homogeneous in an indoor space and can be easier monitored by strategically

positioned sensors.


4/12

An example of each type of real-time air quality monitors is given in the following:-

The YES205 Air Quality Monitor (Figure 1) is a highly compact instrumentfor continuous and simultaneous monitoring of 5 Primary IAQ Parameters

with data logging capacity of 15 days based on 5-minutes sampling intervals,on all 5 parameters. The instrument has a precision calibrated NTC thermister

for temperature sensing, a capacitive polymer sensor for the measurement ofRH, a dual beam absorption infrared sensor for the measurement of CO2, andelectro-chemical sensors for the measurement of CO and NO 2. These

intelligent sensors, coupled with a micro-processor and proprietary software,provide reliable, reproducible and accurate data for evaluation, assessment and

control of air quality parameters. The instrument is equipped with high levelCO2 alarm signal for integral air condition control.

The Formademeter 400 (Figure 2) is an ultra-compact instrument withmicroprocessor control, electrochemical formaldehyde sensor, sampling pump

and digital LCD display. Linking up with a monitor station, it is a continuous

monitor with full data logging and alarm capabilities. The instrument isdesigned to accurately monitor formaldehyde at the levels below 400 ppb, and

has a built-in programme to read and compute parameters such as peakconcentration, short-term average exposure (STEL) and 8-hour time weighed

average (TWA) offering complete automation from sampling to presentationof air quality statistics.

ppbRAE TVOC Monitor (Figure 3) is a compact instrument withmicroprocessor control using photo-ionizations detector to detect a wide range

of organic vapours. Organic vapours passing through a UV lamp are photo-ionized and the ejected electrons are detected as current. The default

calibration gas is Isobutylene.

TSI Dust Trak RSP Monitor (Figure 4) is a compact laser particle counter.

Use the data logging features to make unattended measurements and to

pinpoint hard-to-locate time-of-day. By attaching proper filters, the countercan select ranges of particle sizes for measurement, e.g. PM10 particles.

RAD 7 Radon Monitor (Figure 5) is a portable monitor adopting themeasurement principle is electrostatic collection of alpha-emitters withspectral analysis. After the initial decay from Radon to Polonium-218, there


5/12

particles from the Polonium-218 decay and ignores all the rest. Thus there is a95% recovery from high readings in 15 minutes.

AIR TREATMENT METHODS FOR AIR QUALITY CONTROL

In real-time air quality control, selected air quality Parameters are monitored and thesignals from the real-time monitors are used to control functioning of the

corresponding air-treatment components of the air-conditioning and ventilationsystems. Some common air-treatment methods being used for improvement of airquality are listed in the following table: -

air quality

Parameter

Possible Air-treatment Methods

1.Temperature Regulate coil temperature and air flow of the AHU

2.RH Regulate coil temperature of the AHU and air-reheat

3.CO2 Increase ventilation by controlling of fresh air intake and airexhaust

4.CO Remove source of pollutant, increase ventilation

5.NO2 Remove source of pollutant, remove pollutant by passing through

wet coils of the AHU.

6.TVOC Remove source of pollutant, increase ventilation

7.Radon Remove source of pollutant, increase ventilation

8.Formaldehyde Remove source of pollutant, increase ventilation, detoxification9.Ozone Remove source of pollutant, isolation, detoxification

10.RSP Air filtering

11.AirborneBacteria

Low temperature and low RH controls

12.AirMovement

Adjust direction of air flow

13. Ventilation

with positiveindoor airpressure

Regulate the air quality difference between the air intake and

exhaust air

The above table shows that real-time monitoring with integrated control of air quality

can be effectively applied to control the levels of different air quality Parameters as


6/12

AN EXAMPLE OF REAL-TIME AIR QUALITY CONTROL

The schematic drawing of a system of air quality control of the Primary Parameters is

depicted in Figure 6. This example has shown the following arrangement: -

1. The temperature signal is used to control the chilled water inlet modulating

valve and air flow of the AHU chilled water coil.

2. The RH signal is used to control the modulating valve of the hot water coil forair-reheat.

3. The CO2, CO and NO2 signals are fed into an OR gate for controlling the

amount of outdoor air in response to the highest demand amongst the threeparameters.

4. The differential pressure signal is used to control the amount of exhaust air.

CONCLUSION

Real-time monitors are used to depict Air Quality Profiles. This enables the changingconcentration levels of various air quality parameters over a time period in an indoor

environment to be seen in graphic form and to allow them to be accurately measured.

Through the use of these real-time monitors, it is now possible to make accuratecontrol of temperature and ventilation of the indoor environment by real-time

monitoring with integrated control of the air-conditioning and ventilation systems.For efficient monitoring of air quality, air quality parameters are sub-divided into

three classes as Primary, Secondary and Tertiary parameters. The functions andreliability of real-time air quality monitors are reviewed. A design drawing is shownto illustrate how Primary Parameters can be used for continuous monitoring and

control of an air conditioning system.

REFERENCE

[1] Wai, K.N., IAQ Profile, paper submitted for the Symposium 2001 on Towards


7/12

Figure 1 Multi-gas Real-time Monitor


8/12

Figure 2 Formaldehyde Real-time Monitor

0.1

0.2

0.3

0.4

0.5

0.6

S.T.E

.L.

/ppm

204-1651-6061

Saved at 11:12:42 AMSaved on 2001/6/8

Samples 480

Start Time 10:23:55Start Date 2001/6/6

Peak STEL / ppm 0.557Projected TWA / ppm 3.93

Peak / ppm 0.575

Average / ppm 0.392Minimum / ppm 0.224

205-1652-6061

Saved at 10:59:35 AMSaved on 2001/6/8

Samples 480

Start Time 10:23:53Start Date 2001/6/6


9/12

Fig. 3 - TVOC Real- time Monitor


10/12

Fig. 4 - Particle Real-time Monitor


11/12


12/12

RealTimeMonitoringwithIntegratedCo

ntrolofAirQualityinBuildings

12

Documents

ASD Air Quality e375